3Com Switch 7750 Configuration Guide Guide

226 CHAPTER 29: MSTP CONFIGURATION
n
■ A port can play different roles in different MSTIs.
■ The role a region edge port plays is consistent with the role it plays in the CIST.
For example, port 1 on switch A in
Figure 56 is a region edge port, and it is a
master port in the CIST. So it is a master port in all MSTIs in the region.
Figure 56 Port roles
Port states
Ports can be in the following three states:
■ Forwarding state: Ports in this state can forward user packets and receive/send
BPDU packets.
■ Learning state: Ports in this state can receive/send BPDU packets.
■ Discarding state: Ports in this state can only receive BPDU packets.
Table 153 lists possible combinations of port states and port roles.
Implementation of MSTP MSTP divides a network into multiple MST regions at Layer 2. The CST is
generated between these MST regions, and multiple spanning trees (or, MSTIs)
can be generated in each MST region. As well as RSTP, MSTP uses configuration
BPDUs to generate spanning trees. The only difference is that the configuration
BPDUs for MSTP carry the MSTP configuration information on the switches.
Tabl e 153 Combinations of port states and port roles
Root/
port/Master
port
Designated
port
Region edge
port
Alternate
port
Backup port
Forwarding √ √ √ - -
Learning √ √ √ - -
Discarding √ √ √ √ √
MST region
C
A
B
D
Port 4
Port 1
Port 2
Connected to the
common root
EdgePort
Master
port
Alternate port
Designated
port
Backup
port
Port 3
Port 5
Port 6
Port
role
Port
state